1. Field of the Invention
The invention relates to stabilizing the active devices of a circuit, e.g., transistors, FETs, etc., for use in thick film packages where the active devices have potential oscillation frequencies that are substantially higher than the operational frequencies of circuits that the active devices are being used in.
2. Description of the Related Art
While the use of thick film technology offers significant size and durability advantages, this technology still presents limitations when it comes to grounding circuit components. Among the most troublesome is the difficulty in mounting an active device on the ceramic substrate where a short length connection to the main D.C. (direct current) ground plane is required. A typical active device such as a bipolar transistor or FET (field effect transistor) utilizes the biasing method shown in FIG. 1. This divider uses R.sub.B and R.sub.S to set the base (gate) voltage. Emitter 12 is connected to emitter (source) bias R.sub.E. Resistor R.sub.E stabilizes transistor 11 current. Current I adjusts itself so that emitter 12 is almost at: ##EQU1##
Therefore, current I is as follows: ##EQU2##
In this configuration, the current is very stable even though certain parameters of the active device, transistor 11, change appreciably with temperature.
A bypass capacitor C.sub.E is required to prevent R.F. (radio frequency) voltage from appearing across R.sub.E. There must be a short, direct path from C.sub.E to ground. The position of the main ground plane is customarily on the underside of a 0.025 inch thick alumina (Al.sub.2 O.sub.3) substrate. Plane A in the diagram represents the underside metallization of the substrate.
Also, as is common in such situations, the active devices are capable of oscillating at frequencies substantially greater than the operational frequency of the circuit.